Question

A train moves at a constant speed of 120 km/h for one kilometre and at 40 kmph for the next one kilometer.what is the average speed of the train.

Answer 1

2xy/x+y =2*120*40/120+40=9600/160=60

Answer 2

Average speed of the train is 60 km/hr.

Solution:

The given journey has two speed for two distances, therefore their average speed of train is calculated as follows,

Given: Total distance for the journey be d = 1 km + 1 km = 2 km

Speed for first 1 km = 120 km/h

Speed for second 1 km = 40 km/h

$Time = \frac{Distance}{Speed}$

$T_{1}=\frac{D_{1}}{S_{1}}=\frac{1}{120}$

And similarly time taken for the second speed = $T_{2}$  

$T_{2}=\frac{D_{2}}{S_{2}}=\frac{1}{40}$

Total time taken for the journey: $T=T_{1}+T_{2}$

$\begin{array}{l}{\mathrm{T}=\frac{1}{120}+\frac{1}{40}} \\ \\{\mathrm{T}=\frac{1}{30}}\end{array}$

Average speed = $\frac{\text { Total distance }}{\text { Total time }}$

Average speed = $\frac{2}{\frac{1}{30}}$  

Average speed = $2 \times 30$

Average speed = 60 km/hr